Spin stand testing systems (spinstands) are used for multiple purposes with respect to HDD disks, including disk characterization, testing, and development. In order to perform such operations, the systems need to provide accurate and precise positioning of read-write heads at various track offsets for the purposes of reading and writing to disks. Additionally, though most measurements on a spinstand write at offset 0, certain measurements can also involve accurate and precise positioning of the head at off-track positions for writing. Accordingly, positioning of a head for spinstands is facilitated by using a servo in combination with servo marks (also known as “servo patterns”) in a closed-loop system. The servo marks are written in small sectors on each disk and are used to accurately position the head at different read or write offsets.
Unfortunately, the quality of the servo patterns can greatly affect the repeatability of some essential measured results from spinstands. In general, the head positioning of a conventional servo in a spinstand (e.g., from Guzik Technical Enterprises of Mountain View, Calif.) varies over a range of read or write offsets and is not repeatable from servo-to-servo write. Furthermore, though approaches to improve head positioning accuracy have been developed (e.g., Guzik Servo Improvement Package), these solutions can suffer from drawbacks such as mechanical limitations that limit accuracy and a significant increase in operation time of the spinstand.
Many recording measurements involving a spinstand use a sequence like (1) write a central track, (2) write additional “aggressor” tracks at off track locations, and (3) assess how much the central track has changed. A servo pattern written on the disk before a sequence of tests is used to position the head at the desired radial location. However, this servo pattern is not perfect. There are variations from one writing of the servo to the next. In addition, the servo system may have a minimum step size which limits where the head can be positioned. Both of these problems degrade the accuracy for writing the “aggressor” tracks at precisely the desired location.